In an image formation process such as electrophotography, a copy or print is formed by developing an electrostatic latent image formed on a photoreceptor having a photoconductive substance disposed thereon by using toner particles containing a colorant and then, fixing the toner image to a transfer material such as paper by heat or pressure.
In a color image forming apparatus, it is the common practice to adopt a process of developing an electrostatic latent image formed on a photoreceptor by using plural colors of toner, transferring the image to an intermediate transfer medium, and transferring and fixing the color image to paper or the like. This process is characterized by that only one photoreceptor is necessary for the formation of an electrostatic latent image a complex optical system is not required for the image formation, and an image can be transferred to any medium that cannot be wound around a cylindrical photoreceptor surface.
When an intermediate transfer medium is used, however, the toner is required to have a high transfer efficiency, because transfer is performed twice, that is, transfer from a photoreceptor to the intermediate transfer medium and transfer from the intermediate transfer medium to a recording medium. The toner which has remained untransferred becomes not only a waste toner but causes a phenomenon such as missing of characters or a portion of the image to be formed and deteriorates the quality of the image.
The transfer efficiency is influenced by various factors. Among them, it is considered that charge properties of a toner have a great influence.
It is the common practice to add a charge control agent in order to keep the charge properties of the toner stable. With a view to overcoming such a problem as easy generation of an oppositely charged toner upon friction charging owing to the irregularities on the surface of the toner, conglobation treatment of the toner for preventing charging defects due to undesirable toner surfaces is proposed (see, for example, Japanese Patent Application Laid-Open No. 181733/1995).
The conglobation treatment of a toner is carried out by heating the toner in the fluid form by hot air to soften its surface. In this step, however, the charge control agent existing in the vicinity of the surface of the toner is exposed to high temperatures and is therefore lost from the toner surface or suffers from quality deterioration through sublimation or oxidative decomposition, which makes the charge properties of the toner uneven and becomes one of the causes for transfer defects such as fog or opposite charge.
Further, of the various factors causing deterioration in the quality of the image formed by an image forming apparatus, the shape of a toner has a great influence. When the toner has irregularities on its surface and its shape is not uniform, the fluidity of the toner becomes insufficient even by the addition of a fluidity improving agent, leading to deterioration in developability, transfer properties and cleanability. Re-use of the toner in a developing step after recovery by cleaning tends to further deteriorate the image quality.
In order to improve fluidity or transfer properties of the toner, the toner surface is smoothened by decreasing the irregularities thereon.
As a method of decreasing the irregularities on the toner surface, it is known to conglobate the toner surface by mechanical or thermal fairing treatment as described above.
The mechanical or thermal fairing treatment is however accompanied with the above-described problem that a synthetic resin used as a binder may be conglobated insufficiently, or when exposed to high temperatures upon heat treatment, charge properties become uneven owing to the sublimation or oxidative decomposition of the charge control agent.
There is proposed a method of preparing a toner by precipitating, from a toner particle-forming composition, particles excellent in sphericity in a liquid without carrying out a thermal fairing step.
As one example, proposed is a toner preparation method called “solution dispersion method”, which comprises mixing a binder resin and a colorant in a solvent immiscible with water, dispersing the mixture in an aqueous medium in the presence of a hydrophilic inorganic dispersing agent to prepare a suspension of particles, and then removing the solvent from the suspension by heating or pressure reduction (see, for example, Japanese Patent Laid-Open No. 15902/1997).
The solution dispersion method, however, involves such a problem that two solvents different in polarity are usually employed and because of the easy dissolution of the charge control agent, which exists in the vicinity of the liquid surface of the toner particles, in the solvent, and hence the effects of the charge control agent becomes insufficient.
Another problem of the above-described method is that as a result of sublimation or oxidative decomposition of the charge control agent in a drying step including heating or pressure reducing procedures after the removal of the solvent, the toner tends to be charged unevenly and cause transfer defects such as fog or opposite charge.
Separately, in order to overcome the above-described problems concerning the charging property unevenness, also proposed is a production process of a toner having an excellent sphericity, which comprises polymerizing monomer components of a binder resin in a liquid to prepare an emulsion of polymer particles having an acidic polar or basic polar group, mixing the emulsion with a colorant, a charge control agent, etc., heating and stirring the mixture to form associated particles of the toner (see, for example, Japanese Patent No. 2537503).
The above-described process is characterized by that a toner having an excellent sphericity can be prepared without any fairing treatment after the production of particles. However, this process is accompanied with such a drawback that among the charge control agent particles, those existing in the vicinity of the surface of the particles and having a great influence on the charge properties of the toner is readily eluted out into a solvent of the emulsion of the polymer.
In addition, for separating the toner particles thus prepared from a liquid, this process goes through a drying step by heating and/or pressure reduction. This drying step is, however, sometimes accompanied with sublimation or oxidative decomposition, which tends to disturb even charging of the toner and becomes a cause of transfer defects such as fog and reverse charge.
Although there may be considered, as a countermeasure, to preliminarily increase the concentration of the charge control agent for compensating the elution thereof into the liquid, but this cannot prevent the charging property from becoming uneven due to the adhesion of the eluted charge control agent onto the surface of the precipitated particles.